Many countries have been developing their own FEP (Feature, Event, Process) lists to formulate radionuclide release scenarios in deep disposal repository of spent nuclear fuels and to assess the safety. The main issue in developing a FEP list is to ensure its completeness and comprehensiveness in examining all plausible scenarios of radionuclide release in a repository of interest. To this end, the NEA International FEP (IFEP) list as a generic reference have been developed and updated through long-term international collaborations. Leading countries advanced in the research field of deep geologic disposal of spent nuclear fuels have comparatively mapped their project-specific FEP (PFEP) lists with the IFEP list. Recently in 2019, NEA has published an updated version of IFEP list (ver. 3.0) which has a different classification system: the IFEP version 3.0 has the five main categories including the waste package, repository, geosphere, biosphere and external factors while the previous IFEP versions were mainly classified into the external, environmental, and contaminant factors. Most leading countries in this field, Finland and Sweden, recently succeeded to obtain the design and/or construction licenses for deep geologic disposal of spent nuclear fuel. Therefore, their PFEP lists should be good benchmark cases to the following countries. However, their PFEP lists have not comparatively mapped with the most recent version of IFEP and thus some gaps may exist in showing completeness and comprehensiveness in comparison to the IFEP version 3.0. In this study, we comparatively map the PFEP lists of Finland and Sweden to the IFEP version 3.0. The comparatively mapped PFEP list could be used as the basis for verifying the comprehensiveness and completeness of the domestic PFEP list currently under development in Korea.

Safety assessment is important for the radioactive waste repositories, and several methods are used to develop scenarios for the management of radioactive waste. The intent of the use of these scenarios is to show how the radio nuclides release can affect the safety of disposal system. It plays an essential role of providing scientific and technical information for performance assessment of safety functions. As important as scenario is, numerous studies for their own scenario development have been conducted in many countries. Scenario development methodology is basically divided into four categories: (1) judgmental, (2) fault/event-tree analysis, (3) simulation, and (4) systematic. Under numerous research, these methods have been developed in ways to strengthen the advantages and make up for the weakness. However, it was hard to find any judgmental or fault/event-tree analysis approach in recent safety assessments since they are not well-systemized and difficult to cover all scenarios. Simulation and systematic approaches are used broadly for their convenience of analyzing needed scenarios. Furthermore, several new methodologies, Process Influence Diagram (PID)/Rock Engineering System (RES)/Hybrid, were developed to reinforce the systematic approach in recent studies. Currently, a government project related to the disposal of spent nuclear fuel is in progress in Korea, and the scenario development for safety case is one of the important tasks. Therefore, it is necessary to identify the characteristics and strengths and weaknesses of the latest scenario development and analysis methods to create a unique methodology for Korea. In this paper, the existing methodologies and cases will be introduced, and the considerations for future scenario development will be summarized by considering those used in the nuclear field other than repository issues. Systematic approach, which is the mostly commonly used method, will be introduced in detail with its use in other countries at the subsequent companion paper entitled ‘Case Study for a Disposal Facility for the Spent Nuclear Fuel’.

In Korea, research on the development of safety case, including the safety assessment of disposal facility for the spent nuclear fuel, is being conducted for long-term management planning. The safety assessment procedure on disposal facility for the spent nuclear fuel heavily involves creating scenarios in which radioactive materials from the repository reach the human biosphere by combining Features, Events and Processes (FEP) that describe processes or events occurring around the disposal area. Meanwhile, the general guidelines provided by the IAEA or top-tier regulatory requirements addressed by each country do not mention detailed methods of ‘how to develop scenarios by combining individual FEPs’. For this reason, the overall frameworks of developing scenarios are almost similar, but their details are quite different depending on situation. Therefore, in order to follow up and clearly analyze the methods of how to develop scenarios, it is necessary to understand and compare case studies performed by each institution. In the previous companion paper entitled ‘Research Status and Trends’, the characteristics and advantages/disadvantages of representative scenario development methods were described. In this paper, which is a next series of the companion papers, we investigate and review with a focus on details of scenario development methods officially documented. In particular, we summarize some cases for the most commonly utilized methods, which are categorized as the ‘systematic method’, and this method is addressed by Process Influence Diagram (PID) and Rock Engineering System (RES). The lessons-learned and insight of these approaches can be used to develop the scenarios for enhanced Korean disposal facility for the spent nuclear fuel in the future.